1. Field of the Invention
The present invention relates in general to equipment which is driven with a battery having a high internal resistance or a secondary battery, and more particularly to equipment which is required to be operated for a long time with the battery or secondary battery.
2. Related Background Art
Conventional equipment is shown in FIG. 4. In FIG. 4, a fuel cell 401 is shown as an example of a battery having a high internal resistance or a secondary battery, and a secondary battery 402 is shown as an example of an electric condenser. As shown in FIG. 4, the equipment includes: the fuel cell 401 for outputting an electric power; a load 104 performing a desired function; the secondary battery 402 for accumulating therein an electric power of the fuel cell 401; a charge control unit 403 for controlling an operation for charging the secondary battery 402 with the electric power of the fuel cell 401; an electric power converter 405 for converting the electric power of the fuel cell 401 and the electric power accumulated in the secondary battery 402 into an electric power with which the load 104 can be operated; and a switch 406 provided in the middle of a path through which the electric power accumulated in the secondary battery 402 is supplied to the electric power converter 405 in order to control a diode 407 for preventing a reverse current from being caused to flow from the electric power converter 405 to the secondary battery 402, and an operation for supplying the electric power accumulated in the secondary battery 402 to the electric power converter 405.
With the above-mentioned configuration, when a quantity of electric power of the fuel cell 401 is more than that required to drive the load 104, the charge control unit 403 charges the secondary battery 402 with an excessive electric power of the fuel cell 401. On the other hand, when a quantity of electric power of the fuel cell 401 is less than that required to drive the load 104, it is possible to make up for a quantity of insufficient electric power with the electric power accumulated in the secondary battery 402 (refer to JP 2002-315224 A (pages 2 and 3, and FIG. 3) for example).
Note that since a voltage of the fuel cell 401 is different from that of the secondary battery 402 in many cases, a DC-DC converter is used as the charge control unit 403.
The above-mentioned conventional equipment is configured so that a maximum electric power is obtained from the fuel cell as the battery having a high internal resistance, and the maximum electric power is used in driving of the load and charging of the secondary battery. In case of adopting of such a configuration, a quantity of current obtained from the fuel cell also becomes large. A problem in the case where a quantity of current obtained from the fuel cell also becomes large as described above will now be described with reference to FIG. 5. FIG. 5 is a graphical representation showing a relationship between an output current from a battery, such as a fuel cell, having a high internal resistance, and a battery voltage and an integration value of an electric power able to be obtained from the battery. As shown in FIG. 5, as the output current is increased, the integration value of the electric power is reduced since the battery voltage is reduced due to a voltage drop developed by causing the output current to flow through the internal resistance of the battery, and an electric power loss is increased due to calorification of the internal resistance of the battery. As a result, in case of the above-mentioned conventional equipment, since a quantity of current obtained from the fuel cell is large, the electric power loss due to flowing of the current through the internal resistance of the fuel cell is increased to reduce the integration value of the electric power able to be obtained from the fuel cell.
That is to say, in case of the configuration of the equipment driven with the battery, such as the above-mentioned conventional fuel cell, having a high internal resistance, there is encountered a problem that since the electric power loss due to calorification of the internal resistance of the battery is increased, the integration value of the electric power able to be obtained from that battery is reduced to shorten an operating time of the equipment.
Recently, the fuel cell has particularly attracted attention owing to its miniature size and large capacity. Thus, an electric automobile, a mobile apparatus, and the like each using a fuel cell have been actively developed. If the above-mentioned problem can be solved, then the realizability of the electric automobile, the mobile apparatus, and the like each driven with the fuel cell will be greatly enhanced.
Incidentally, when the maximum electric power is obtained from the battery having a high internal resistance or the secondary battery, a current has to be obtained from the battery so that a voltage drop due to the internal resistance of the battery becomes about half an open voltage of the battery. In this case, about 50% of the electric power is consumed in the form of calorification of the internal resistance, and hence the integration value of the electric power obtained from the battery is reduced to about half that when a current is hardly obtained from the battery. Consequently, the duration of the above-mentioned conventional equipment is nearly halved.
In addition, in the fuel cell of a direct methanol system (hereinafter referred to as “a DMFC system” for short when applicable) which has attracted attention recently, a portion of the fuel is lost with time due to methanol crossover without being converted into an electric power. That is to say, the self-discharge is very large. FIG. 6 is a graphical representation showing a relationship between an output current obtained from a battery such as a fuel cell of the DMFC system having large self-discharge and a high internal resistance or a secondary battery, and a battery voltage and an integration value of an electric power able to be obtained from the battery. As shown in FIG. 6, when a quantity of current obtained from the battery is small, the loss due to the self-discharge is increased, while when a quantity of current obtained from the battery is large, the electric power loss is increased due to the calorification of the internal resistance of the battery. Hence, the integration value of the electric power able, to be obtained from the battery becomes maximum at a certain value of the obtained current.
That is to say, in such a battery, if such a current as to make the integration value of the obtained electric power maximum is obtained, then the electric power of the battery can be most efficiently utilized. However, as described above, in case of the configuration of the above-mentioned conventional equipment, there is encountered a problem that since the electric power loss due to the calorification of the internal resistance of the battery is increased, the integration value of the electric power able to be obtained from the battery is reduced to shorten the operating time of the equipment.